1. Field of the Invention
The invention relates to the field of recording and/or replaying standard video data using a video recorder.
2. Description of the Related Art
It is well known that it is not possible to reliably record and replay teletext data using a standard VHS video cassette recorder, principally because the bandwidth of the VHS channel is very limited. The luminance bandwidth is 2.9-3 MHz which can be reduced to 2 MHz on playback when the sharpness control is set to "soft". As a comparison the bandwidth of a normal broadcast video channel is of the order of 5.5 MHz. The limited VHS bandwidth has proved acceptable with normal video signals, the effect being to soften or blur the picture to an extent which has proved to be acceptable to most viewers. When teletext was first specified, however, a high bit rate was chosen to make maximum use of the broadcast bandwidth. Unfortunately, this means that the high frequency part of the data signal, including the very important clock run-in signal, is obliterated by the recording and replay process. In addition most teletext decoders rely on the very close line timing tolerances achieved by broadcast transmissions. The decoders can be completely upset by synchronising jumps created just before the vertical blanking interval (VBI) by the recorder head switching. Further problems are caused by the variation in line frequency caused by stretched tapes which can cause a variation of .+-.3% in line frequency.
It has been proposed in EP-A-0 608 960 (PHB 33832), to U.S. Pat. No. 5,565,997, issued Oct. 15, 1996, that teletext data should be recorded on VHS tapes at a lower bit rate by decoding the teletext data and expanding the data packets by a factor of four. This data is then spread across five VBI lines, to allow for guard periods. On replay the data packets from the five lines are compressed to restore the data rate to that in the teletext specification to enable display of the data.
The above citations are hereby incorporated herein in whole by reference. Such an arrangement enables the teletext data to be recorded on and replayed from a VHS recorder but suffers from the disadvantage that it is impossible to record all the teletext pages that are transmitted as there are insufficient VBI lines to accommodate them. Consequently a selection of the pages to be recorded has to be made at the time of recording and as a result only those selected pages can be displayed when the tape is replayed.
A further proposal in GB-A-2132856 is that the teletext data be written into a buffer store at the standard teletext data rate and read out at half that rate for recording on tape. In this instance no selection of pages to be stored is made, but correct operation depends on there being at least twice as many VBI lines available as are used for teletext data transmission. This is not currently the case in the UK where more than half the available VBI lines contain teletext data or other data.